Add gosu back

Author: mkuratczyk

packaging/docker-image/Dockerfile

@@ -29,6 +29,7 @@ RUN set -eux; \
 	apt-get update; \
 	apt-get install --yes --no-install-recommends \
 		ca-certificates \
+		gosu \
 		tzdata \
 		gnupg \
 		wget \